Pitch-based carbon foam heat sink with phase change material

ABSTRACT

A process for producing a carbon foam heat sink is disclosed which obviates the need for conventional oxidative stabilization. The process employs mesophase or isotropic pitch and a simplified process using a single mold. The foam has a relatively uniform distribution of pore sizes and a highly aligned graphic structure in the struts. The foam material can be made into a composite which is useful in high temperature sandwich panels for both thermal and structural applications. The foam is encased and filled with a phase change material to provide a very efficient heat sink device.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of an earlier filedU.S. patent application-Ser. No. 08/921,875, filed on Sep. 2, 1997, andU.S. patent application Ser. No. 08/923,877, filed Sep. 2, 1997, both ofwhich are herein incorporated in their entirety by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

[0002] The United States Government has rights in this inventionpursuant to contract No. DE-AC05-960R22464 between the United StatesDepartment of Energy and Lockheed Martin Energy Research Corporation.

BACKGROUND OF THE INVENTION

[0003] The present invention relates to porous carbon foam filled withphase change materials and encased to form a heat sink product, and moreparticularly to a process for producing them.

[0004] There are currently many applications that require the storage oflarge quantities of heat for either cooling or heating an object.Typically these applications produce heat so rapidly that normaldissipation through cooling fins, natural convection, or radiationcannot dissipate the heat quickly enough and, thus, the object overheats. To alleviate this problem, a material with a large specific heatcapacity, such as a heat sink, is placed in contact with the object asit heats. During the heating process, heat is transferred to the heatsink from the hot object, and as the heat sink's temperature rises, it“stores” the beat more rapidly than can be dissipated to the environmentthrough convection. Unfortunately, as the temperature of the heat sinkrises the heat flux from the hot object decreases, due to a smallertemperature difference between the two objects. Therefore, although thismethod of energy storage can absorb large quantities of heat in someapplications, it is not sufficient for all applications

[0005] Another method of absorbing heat is through a change of phase ofthe material, rather than a change in temperature. Typically, the phasetransformation of a material absorbs two orders of magnitude greaterthermal energy than the heat capacity of the material. For example, thevaporization of 1 gram of water at 100° C. absorbs 2,439 joules ofenergy, whereas changing the temperature of water from 99° C. to 100° C.only absorbs 4.21 Joules of energy. In other words, raising thetemperature of 579 grams of water from 99° C. to 100° C. absorbs thesame amount of heat as evaporating 1 gram of water at 100° C. The sametrend is found at the melting point of the material. This phenomenon hasbeen utilized in some applications to either absorb or evolve tremendousamounts of energy in situations where heat sinks will not work.

[0006] Although a solid block of phase change material has a very largetheoretical capacity to absorb heat, the process is not a rapid onebecause of the difficulties of heat transfer and thus it cannot beutilized in certain applications. However, the utilization of the highthermal conductivity foam will overcome the shortcomings describedabove. If the high conductivity to foam is filled with the phase changematerial, the process can become very rapid. Because of the extremelyhigh conductivity in the struts of the foam, as heat contacts thesurface of the foam, it is rapidly transmitted throughout the foam to avery large surface area of the phase change material. Thus, heat is veryquickly distributed throughout the phase change material, allowing it toabsorb or emit thermal energy extremely quickly without changingtemperature, thus keeping the driving force for heat transfer at itsmaximum.

[0007] Heat sinks have been utilized in the aerospace community toabsorb energy in applications such as missiles and aircraft where rapidheat generation is found. A material that has a high heat of melting isencased in a graphite or metallic case, typically aluminum, and placedin contact with the object creating the heat. Since most phase changematerials have a low thermal conductivity, the rate of heat transferthrough the material is limited, but this is offset by the high energyabsorbing capability of the phase change. As heat is transmitted throughthe metallic or graphite case to the phase change material, the phasechange material closest to the heat source begins to melt. Since thetemperature of the phase change material does not change until all thematerial melts, the flux from the heat source to the phase changematerial remains relatively constant. However, as the heat continues tomelt more phase change material, more liquid is formed. Unfortunately,the liquid has a much lower thermal conductivity, thus hampering heatflow further. In fact, the overall low thermal conductivity of the solidand liquid phase change materials limits the rate of heat absorptionand, thus, reduces the efficiency of the system.

[0008] Recent developments of fiber-reinforced composites, includingcarbon foams, have been driven by requirements for improved strength,stiffness, creep resistance, and toughness in structural engineeringmaterials. Carbon fibers have led to significant advancements in theseproperties in composites of various polymeric, metal, and ceramicmatrices.

[0009] However, current applications of carbon fibers have evolved fromstructural reinforcement to thermal management in application rangingfrom high-density electronic modules to communication satellites. Thishas stimulated research into novel reinforcements and compositeprocessing methods. High thermal conductivity, low weight, and lowcoefficient of thermal expansion are the primary concerns in thermalmanagement applications. See Shih, Wei, “Development of Carbon-CarbonComposites for Electronic Thermal Management Applications,” IDAWorkshop, May 3-5, 1994, supported by AF Wright Laboratory underContract Number F33615-93-C-2363 and AR Phillips Laboratory ContractNumber F29601-93C-0165 and Engle, G. B., “High Thermal Conductivity C/CComposites for Thermal Management,” IDA Workshop, May 3-5, 1994,supported by AF Wright Laboratory under Contract F33615-93-C-2363 and ARPhillips Laboratory Contract Number F29601-93-C-0165. Such applicationsare striving towards a sandwich type approach in which a low-densitystructural core material (i.e. honeycomb or foam) is sandwiched betweena high thermal conductivity facesheet. Structural cores are limited tolow density materials to ensure that the weight limits are not exceeded.Unfortunately, carbon foams and carbon honeycomb materials are the onlyavailable materials for use in high temperature applications (>1600°C.). High thermal conductivity carbon honeycomb materials are extremelyexpensive to manufacture compared to low conductivity honeycombs,therefore, a performance penalty is paid for low cost materials. Highconductivity carbon foams are also more expensive to manufacture thanlow conductivity carbon foams, in part, due to the starting materials.

[0010] In order to produce high stiffness and high conductivity carbonfoams, invariably, a pitch must be used as the precursor. This isbecause pitch is the only precursor which forms a highly alignedgraphitic structure which is a requirement for high conductivity.Typical processes utilize a blowing technique to produce a foam of thepitch precursor in which the pitch is melted and passed from a highpressure region to a low pressure region. Thermodynamically, thisproduces a “Flash,” thereby causing the low molecular weight compoundsin the pitch to vaporize (the pitch boils), resulting in a pitch foam.See Hagar, Joseph W. and Max L. Lake, “Novel Hybrid Composites Based onCarbon Foams,” Mat. Res. Soc. Symp ., Materials Research Society,270:29-34 (1992); Hagar, Joseph W. and Max L. Lake, “Formulation of aMathematical Process Model Process Model for the Foaming of a MesophaseCarbon Precursor,” Mat. Res. Soc. Symp., Materials Research Society,270:3540 (1992); Gibson, L. J. and M. F. Ashby, Cellular Solids:Structures & Properties, Pergamon Press, New York (1988); Gibson, L. J.,Mat. Sci. and Eng A110, 1 (1989); Knippenberg and B. Lersmacher,Phillips Tech. Rev., 36(4), (1976); and Bonzom, A., P. Crepaux and E. J.Moutard, U.S. Pat. No. 4,276,246, (1981). Then, the pitch-foam must beoxidatively stabilized by heating in air (or oxygen) for many hours,thereby, crosslinking the structure and “stabilizing” the pitch so itdoes not melt during carbonization. See Hagar, Joseph W. and Max L.Lake, “Formulation of a Mathematical Process Model Process Model for theFoaming of a Mesophase Carbon Precursor, Mat. Res. Soc. Symp., MaterialsResearch to Society, 270:35-40 (1992); and White, J. L., and P.M.Shaeffer. Carbon, 27:697 (1989). This is a time consuming step and canbe an expensive step depending on the part size and equipment required.The “stabilized” or oxidized pitch is then carbonized in an inertatmosphere to temperatures as high as 1100° C. Then, graphitization isperformed at temperatures as high as 3000° C. to produce a high thermalconductivity graphitic structure, resulting in a stiff and verythermally conductive foam.

[0011] Other techniques utilize a polymeric precursor, such as phenolic,urethane, or blends of these with pitch. See Hagar, Joseph W. and Max L.Lake, “Idealized Strut Geometries for Open-Celled Foams,” Mat. Res. Soc.Symp., Materials Research Society, 270:4146 (1992); Aubert, J. W., MRSSymposium Proceedings, 207:117-127 (1990); Cowlard, F. C. and J. C.Lewis, J. of Mat. Sci., 2:507-512 (1967); and Noda, T., Inagaki and S.Yamada, J. of Non-Crystalline Solids, 1:285-302, (1969). High pressureis applied and the sample is heated. At a specified temperature, thepressure is released, thus causing the liquid to foam as volatilecompounds are released. The polymeric precursors are cured and thencarbonized without a stabilization step. However, these precursorsproduce a “glassy” or vitreous carbon which does not exhibit graphiticstructure and, thus, has low thermal conductivity and Low stiffness. SeeHagar, Joseph W. and Max L. Lake, “Idealized Strut Geometries forOpen-Celled Foams,” Mat. Res. Soc. Symp., Materials Research Society,270:41-46 (1992).

[0012] In either case, once the foam is formed, it is then bonded in aseparate step to the facesheet used in the composite. This can be anexpensive step in the utilization of the foam.

[0013] The extraordinary mechanical properties of commercial carbonfibers are due to the unique graphitic morphology of the extrudedfilaments. See Edie, D. D., “Pitch and Mesophase Fibers,” in CarbonFibers, Filaments and Composites, Figueiredo (editor), Kluwer AcademicPublishers, Boston, pp. 43-72 (1990). Contemporary advanced structuralcomposites exploit these properties by creating a disconnected networkof graphitic filaments held together by an appropriate matrix. Carbonfoam derived from a pitch precursor can be considered to be aninterconnected network of graphitic ligaments, or struts, as shown inFIG. 1. As such interconnected networks, they represent a potentialalternative as reinforcement in structural composite materials.

[0014] The process of this invention overcomes current manufacturinglimitations by avoiding a “blowing” or “pressure release” technique toproduce the foam. Furthermore, an oxidation stabilization step is notrequired, as in other methods used to produce pitch based carbon foamswith a highly aligned graphitic structure. This process is less timeconsuming, and therefore, will be lower in cost and easier to fabricate.The foam can be produced with an integrated sheet of high thermalconductivity carbon on the surface of the foam, thereby producing acarbon foam with a smooth sheet on the surface to improve heat transfer.

SUMMARY OF THE INVENTION

[0015] An object of the present invention is production of encased highthermal conductivity porous carbon foam filled with a phase changematerial wherein tremendous amounts of thermal energy are stored andemitted very rapidly. The porous foam is filled with a phase changematerial (PCM) at a temperature close to the operating temperature ofthe device. As heat is added to the surface, from a heat source such asa computer chip, friction due to re-entry through the atmosphere, orradiation such as sunlight, it is transmitted rapidly and uniformlythroughout the foam and then to the phase change material. As thematerial changes phase, it absorbs orders of magnitude more energy thannon-PCM material due to transfer of the latent heat of fusion orvaporization. Conversely, the filled foam can be utilized to emit energyrapidly when placed in contact with a cold object.

[0016] Non-limiting embodiments disclosed herein are a device to rapidlythaw frozen foods or freeze thawed foods, a design to preventoverheating of satellites or store thermal energy as they experiencecyclic heating during orbit, and a design to cool leading edges duringhypersonic flight or re-entry from space.

[0017] Another object of the present invention is to provide carbon foamand a composite from a mesophase or isotropic pitch such as synthetic,petroleum or coal-tar based pitch. Another object is to provide a carbonfoam and a composite from pitch which does not require an oxidativestabilization step.

[0018] These and other objectives are accomplished by a method ofproducing carbon foam heat sink wherein an appropriate mold shape isselected and preferably an appropriate mold release agent is applied towalls of the mold. Pitch is introduced to an appropriate level in themold, and the mold is purged of air by applying a vacuum, for example.Alternatively, an inert fluid could be employed. The pitch is heated toa temperature sufficient to coalesce the pitch into a liquid whichpreferably is of about 50° C. to about 100° C. above the softening pointof the pitch. The vacuum is released and an inert fluid applied at astatic pressure up to about 1000 psi. The pitch is heated to atemperature sufficient to cause gases to evolve and foam the pitch. Thepitch is further heated to a temperature sufficient to coke the pitchand the pitch is cooled to room temperature with a simultaneous andgradual release of pressure. The foam is then filled with a phase changematerial and encased to produce an efficient heat storage product.

[0019] In another aspect, the previously described steps are employed ina mold composed of a material such that the molten pitch does not adhereto the surface of the mold.

[0020] In yet another aspect, the objectives are accomplished by thecarbon foam product produced by the methods disclosed herein including afoam product with a smooth integral facesheet.

[0021] In still another aspect a carbon foam composite product isproduced by adhering facesheets to the carbon foam produced by theprocess of this invention.

[0022]FIG. 1 is section cut of a heat sink device for thawing food usingacetic acid as the phase change material.

[0023]FIG. 2 is a section cut of a heat sink to prevent overheating ofsatellites during cyclic orbits.

[0024]FIG. 3 is a section cut of a heat sink used on the leading edge ofa shuttle orbiter.

[0025]FIG. 4 is a micrograph illustrating typical carbon foam withinterconnected carbon ligaments and open porosity.

[0026]FIG. 5-9 are micrographs of pitch-derived carbon foam graphitizedat 2500° C. and at various magnifications.

[0027]FIG. 10 is a SEM micrograph of carbon foam produced by the processof this invention.

[0028]FIG. 11 is a chart illustrating cumulative intrusion volume versuspore diameter.

[0029]FIG. 12 is a chart illustrating log differential intrusion volumeversus pore diameter.

[0030]FIG. 13 is a graph illustrating the temperatures at whichvolatiles are given off from raw pitch.

[0031]FIG. 14 is an X-ray analysis of the graphitized foam produced bythe process of this invention.

[0032] FIGS. 15 A-C are photographs illustrating foam produced withaluminum crucibles and the smooth structure or face sheet that develops.FIG. 16A is a schematic view illustrating the production of a carbonfoam composite made in accordance with this invention.

[0033]FIG. 16B is a perspective view of the carbon foam composite ofthis invention.

DETAILED DESCRIPTION OF THE INVENTION

[0034] In order to illustrate the carbon foam heat sink product of thisinvention, the following examples are set forth. They are not intendedto limit the invention in any way.

EXAMPLE 1

[0035] Device for Thawing Food

[0036] Acetic acid has a heat of melting of 45 J/g at a melting point of11° C. The heat of melting of food, primarily ice, is roughly 79 J/g at0° C. Therefore, take a block of foam and fill it with liquid aceticacid at room temp. The foam will be encased in a box made from aninsulating polymer such as polyethylene on all sides except the top. Thetop of the foam/acetic acid block will be capped with a high thermalconductivity aluminum plate that snaps into place thus sealing thefoam/acetic acid inside the polymer case (illustrated in FIG. 1). If thefoam block is 10-in.×15-in.×0.5-in. thick, the mass of foam is 614grams. The mass of acetic acid that fills the foam is roughly 921 grams.Therefore, when a piece of frozen meat is placed in contact with the topof the aluminum block, the foam will coot to the freezing point of theacetic acid (11° C.). At this point, the heat given off from the aceticacid as it freezes (it also remains at 11° C.) will be equivalent to 49KJ. This heat is rapidly transferred to the frozen meat as it thaws (italso remains at 0° C.). This amount of heat is sufficient to meltroughly 500 grams (1 lb.) of meat.

EXAMPLE 2

[0037] Heat Sink To Prevent Overheating of Satellites During CyclicOrbits.

[0038] Produce a carbon-carbon composite with the foam in which the foamis a core material with carbon-carbon face sheets (FIG. 2). Fill thefoam core with a suitable phase change material, such as a paraffin wax,that melts around the maximum operating temperature of the satellitecomponents. One method to perform this would be to drill a hole in onesurface of the carbon-carbon face sheets and vacuum fill the phasechange material in the liquid state into the porous foam. Once filled,the sample can be cooled (the phase change material solidifies) and thehole can be plugged with an epoxy or screw-type cap. The epoxy and anyother sealant must be able to withstand the operating temperature of theapplication. The foam-core composite will to then be mounted on the sideof the satellite that is exposed to the sun during orbit. As thesatellite orbits the earth and is exposed to the sun, the radiant energyfrom the sun will begin to heat the composite panel to the melting pointof the phase change material. At this point, the panel will not increasein temperature as the phase change material melts. The amount of radiantenergy the panel can absorb will be dependent on the thickness and outerdimensions of the panel. This can be easily calculated and designedthrough knowledge of the orbit times of the satellite such that thematerial never completely melts and, thus, never exceeds the melttemperature. Then, when the satellite leaves the view of the sun, itwill begin to radiate heat to space and the phase change material willbegin to freeze. The cycle will repeat itself once the satellite comesinto view of the sun once again.

EXAMPLE 3

[0039] Heat Sink for Leading Edges

[0040] Currently, the shuttle orbiter experiences extreme heats duringreentry. Specifically, the leading edges of the craft can reach 1800° C.and the belly of the craft can reach temperatures as high as 1200° C. Ifa foam core composite panel is placed at the surface of the leadingedges and at the surface of the belly (FIG. 3), it would be able toabsorb enough energy to dramatically reduce the maximum temperature ofthe hot areas. This also would permit a faster re-entry or (steeperglide slope) and maintain the current maximum temperatures. In this casethe phase change material would most likely be an alloy, e.g.germanium-silicon, which melts around 800-900C and does not vaporizeuntil much higher than the maximum temperature of the craft.

[0041] For example, Germanium has a heat of formation (heat of melting)of 488 J/g. This would require 1.0 Kg of Germanium to reduce thetemperature of 1 Kg of existing carbon/carbon heat-shield by 668° C. Inother words, if the existing carbon-carbon were replaced pound-for-poundwith germanium filled foam, the maximum temperature of the heat shieldwould only be about 1131° C. instead of about 1800° C. during re-entry,depending on the duration of thermal loading.

EXAMPLE 4

[0042] Pitch powder, granules, or pellets are placed in a mold with thedesired final shape of the foam. These pitch materials can be solvatedif desired. In this example, Mitsubishi ARA-24 mesophase pitch wasutilized. A proper mold release agent or film is applied to the sides ofthe mold to allow removal of the part. In this case, Boron Nitride sprayand Dry Graphite Lubricant were separately used as a mold release agent.If the mold is made from pure aluminum, no mold release agent ISnecessary since the molten pitch does not adhere to the aluminum and,thus, will not stick to the mold. Similar mold materials may be foundthat the pitch does not adhere and, thus, they will not need moldrelease. The sample is evacuated to less than 1 torr and then heated toa temperature approximately 50 to 100° C. above the softening point. Inthis case where Mitsubishi ARA24 mesophase pitch was used, 300° C. wassufficient. At this point, the vacuum is released to a nitrogen blanketand then a pressure of up to 1000 psi is applied. The temperature of thesystem is then raised to 800° C., or a temperature sufficient to cokethe pitch which is 500° C. to 1000° C. This is performed at a rate of nogreater than 5° C./min. and preferably at about 2° C./min. Thetemperature is held for at least 15 minutes to achieve an assured soakand then the furnace power is turned off and cooled to room temperature.Preferably the foam was cooled at a rate of approximately 1.5° C./min.with release of pressure at a rate of approximately 2 psi/min. Finalfoam temperatures for three product runs were 500° C., 630° C. and 800°C. During the cooling cycle, pressure is released gradually toatmospheric conditions. The foam was then heat treated to 1050° C.(carbonized) under a nitrogen blanket and then heat treated in separateruns to 2500° C. and 2800° C. (graphitized) in Argon.

[0043] Carbon foam produced with this technique was examined withphotomicrography, scanning electron microscopy (SEM), X-ray analysis,and mercury porisimetry. As can be seen in the FIGS. 5-10, theisochromatic regions under cross-polarized light indicate that thestruts of the foam are completely graphitic. That is, all of the pitchwas converted to graphite and aligned along the axis of the struts.These struts are also similar in size and are interconnected throughoutthe foam. This would indicate that the foam would have high stiffnessand good strength. As seen in FIG. 10 by the SEM micrograph of the foam,the foam is open cellular meaning that the porosity is not closed. FIGS.11 and 12 are results of the mercury porisimetry tests. These testsindicate that the pore sizes are in the range of 90-200 microns.

[0044] A thermogravimetric study of the raw pitch was performed todetermine the temperature at which the volatiles are evolved. As can beseen in FIG. 14, the pitch loses nearly 20% of its mass fairly rapidlyin the temperature range between about 420° C. and about 480° C.Although this was performed at atmospheric pressure, the addition of1000 psi pressure will not shift this effect significantly. Therefore,while the pressure is at 1000 psi, gases rapidly evolved during heatingthrough the temperature range of 420° C. to 480° C. The gases produce afoaming effect (like boiling) on the molten pitch. As the temperature isincreased further to temperatures to ranging from 500° C. to 1000° C.(depending on the specific pitch), the foamed pitch becomes coked (orrigid), thus producing a solid foam derived from pitch. Hence, thefoaming has occurred before the release of pressure and, therefore, thisprocess is very different from previous art.

[0045] Samples from the foam were machined into specimens for measuringthe thermal conductivity. The bulk thermal conductivity ranged from 58W/m·K to 106 W/m·K. The average density of the samples was 0.53 g/cm³.When weight is taken into account, the specific thermal conductivity ofthe pitch derived from foam is over 4 times greater than that of copper.Further derivations can be utilized to estimate the thermal conductivityof the struts themselves to be nearly 700 W/m·K. This is comparable tohigh thermal conductivity carbon fibers produced from this same ARA24mesophase pitch.

[0046] X-ray analysis of the foam was performed to determine thecrystalline structure of the material. The x-ray results are shown inFIG. 14. From this data, the graphene layer spacing (d₀₀₂) wasdetermined to be 0.336 nm. The coherence length (L_(a,100)) wasdetermined to be 203.3 nm and the stacking height was determined to be442.3 nm.

[0047] The compression strength of the samples were measured to be 3.4MPa and the compression modulus was measured to be 73.4 MPa. The foamsample was easily machined and could be handled readily without fear ofdamage, indicating good strength.

[0048] It is important to note that when this pitch is heated in asimilar manner, but only under atmospheric pressure, the pitch foamsdramatically more than when under pressure. In fact, the resulting foamis so fragile that it could not even be handled to perform tests.Molding, under pressure serves to limit the growth of the cells andproduces a usable material.

EXAMPLE 5

[0049] An alternative to the method of Example 4 is to utilize a moldmade from aluminum. In this case two molds were used, an aluminumweighing dish and a sectioned soda can. The same process as set forth inExample 4 is employed except that the final coking temperature was only630° C., so as to prevent the aluminum from melting.

[0050] FIGS. 15 A-C illustrate the ability to utilized complex shapedmolds for producing complex shaped foam. In one case, shown in FIG. 15A,the top of a soda can was removed and the remaining can used as a mold.No release agent was utilized. Note that the shape of the resulting partconforms to the shape of the soda can, even after graphitization to2800° C. This demonstrates the dimensional stability of the foam and theability to produce near net shaped parts.

[0051] In the second case, as shown in FIGS. 15B and C employing analuminum weight dish, a very smooth surface was formed on the surfacecontacting the aluminum. This is directly attributable to the fact thatthe molten pitch does not adhere to the surface of the aluminum. Thiswould allow one to produce complex shaped parts with smooth surfaces soas to improve contact area for bonding or improving heat transfer. Thissmooth surface will act as a face sheet and, thus, a foam-core compositecan be fabricated in-situ with the fabrication of the face sheet. Sinceit is fabricated together and an integral material no interface jointsresult, thermal stresses will be less, resulting in a stronger material.

[0052] The following examples illustrate the production of a compositematerial employing the foam of this invention.

EXAMPLE 6

[0053] Pitch derived carbon foam was produced with the method describedin Example 4. Referring to FIG. 16A the carbon foam 10 was then machinedinto a block 2″×2″{fraction (1/2″)}. Two pieces 12 and 14 of a prepegcomprised of Hercules AS4 carbon fibers and ICI FibiritePolyetheretherkeytone thermoplastic resin also of 2″×2″×{fraction(1/2″)} size were placed on the top and bottom of the foam sample, andall was placed in a matched graphite mold 16 for compression by graphiteplunger 18. The composite sample was heated under an applied pressure of100 psi to a temperature of 380° C. at a rate of 5° C./min. Thecomposite was then heated under a pressure of 100 psi to a temperatureof 650° C. The foam core sandwich panel generally 20 was then removedfrom the mold and carbonized under nitrogen to 1050° C. and thengrapbitized to 2800° C., resulting in a foam with carbon-carbonfacesheets bonded to the surface. The composite generally 30 is shown inFIG. 16B.

EXAMPLE 7

[0054] Pitch derived carbon foam was produced with the method describedin Example 4. It was then machined into a block 2″×2″×{fraction (1/2″)}.Two pieces of carbon-carbon material, 2″×2″×{fraction (1/2″)}, werecoated lightly with a mixture of 50% ethanol, 50% phenolic Durez© Resinavailable from Occidental Chemical Co. The foam block and carbon-carbonmaterial were positioned together and placed in a mold as indicated inExample 6. The sample was heated to a temperature of 150° C. at a rateof 5° C./min and soaked at temperature for 14 hours. The sample was thencarbonized under nitrogen to 1050° C. and then graphitized to 2800° C.,resulting in a foam with carbon-carbon facesheets bonded to the surface.This is also shown generally at 30 in FIG. 16B.

EXAMPLE 8

[0055] Pitch derived carbon foam was produced with the method describedin Example 4. The foam sample was then densified with carbon by themethod of chemical vapor infiltration for 100 hours. The densityincreased to 1.4 g/cm³, the flexural strength was 19.5 MPa and theflexural modulus was 2300 MPa. The thermal conductivity of the raw foamwas 58 W/m·K and the thermal conductivity of the densified foam was 94W/m·K.

EXAMPLE 9

[0056] Pitch derived carbon foam was produced with the method describedin Example 4. The foam sample was then densified with epoxy by themethod of vacuum impregnation. The epoxy was cured at 150° C. for 5hours. The density increased to 1.37 g/cm³ and the flexural strength wasmeasured to be 19.3 MPa.

[0057] Other possible embodiments may include materials, such as metals,ceramics, plastics, or fiber-reinforced plastics bonded to the surfaceof the foam of this invention to produce a foam core composite materialwith acceptable properties. Additional possible embodiments includeceramics, glass, or other materials impregnated into the foam fordensification.

[0058] Based on the data taken to date from the carbon foam material,several observations can be made outlining important features of theinvention that include:

[0059] 1. Pitch-based carbon foam can be produced without an oxidativestabilization step, thus saving time and costs.

[0060] 2. High graphitic alignment in the struts of the foam is achievedupon graphitization to 2500° C., and thus high thermal conductivity andstiffness will be exhibited by the foam, making them suitable as a corematerial for thermal-applications.

[0061] 3. High compressive strengths should be achieved with mesophasepitch-based carbon foams, making them suitable as a core material forstructural applications.

[0062] 4. Foam core composites can be fabricated at the same time as thefoam is generated, thus saving time and costs.

[0063] 5. Rigid monolithic preforms can be made with significant openporosity suitable for densification by the Chemical Vapor Infiltrationmethod of ceramic and carbon infiltrants.

[0064] 6. Rigid monolithic preforms can be made with significant openporosity suitable for activation, producing a monolithic activatedcarbon.

[0065] 7. It is obvious that by varying the pressure applied, the sizeof the bubbles formed during the foaming will change and, thus, thedensity, strength, and other properties can be affected.

[0066] The following alternative procedures and products can also beeffected by the process of this invention:

[0067] 1. Fabrication of preforms with complex shapes for densificationby CVI or Melt Impregnation.

[0068] 2. Activated carbon monoliths with high thermal conductivity.

[0069] 3. Optical absorbent.

[0070] 4. Low density heating elements.

[0071] 5. Firewall Material

[0072] 6. Low secondary electron emission targets for high-energyphysics applications.

[0073] The present invention provides for the manufacture of pitch-basedcarbon foam heat sink for structural and thermal composites. The processinvolves the fabrication of a graphitic foam from a mesophase orisotropic pitch which can be synthetic, petroleum, or coal-tar based. Ablend of these pitches can also be employed. The simplified processutilizes a high pressure high temperature furnace and thereby, does notrequire and oxidative stabilization step. The foam has a relativelyuniform distribution of pore sizes (−100 microns), very little closedporosity, and density of approximately 0.53 g/cm³. The mesophase pitchis stretched along the struts of the foam structure and thereby producesa highly aligned graphitic structure in the struts. These struts willexhibit thermal conductivities and stiffness similar to the veryexpensive high performance carbon fibers (such as P-120 and K1100).Thus, the foam will exhibit high stiffness and thermal conductivity at avery low density (−0.5 g/cc). This foam can be formed in place as a corematerial for high temperature sandwich panels for both thermal a,idstructural applications, thus reducing fabrication time. By utilizing anisotropic pitch, the resulting foam can be easily activated to produce ahigh surface area activated carbon. The activated carbon foam will notexperience the problems associated with granules such as attrition,channeling, and large pressure drops.

What is claimed is:
 1. A process of producing a carbon foam heat sinkcomprising: selecting an appropriate mold shape; introducing pitch to anappropriate level in said mold; purging air from said mold to form avacuum; heating said pitch to a temperature sufficient to coalesce saidpitch into a liquid; releasing said vacuum and backfilling an inertfluid at a static pressure up to about 1000 psi; heating said pitch to atemperature sufficient to cause gases to evolve and form a carbon foam;heating said carbon foam to a temperature sufficient to coke the pitch;cooling said carbon foam to room temperature and simultaneouslyreleasing said inert fluid; at least partially encasing said carbonfoam; and at least partially filling porous regions of said carbon foamwith a phase change material.
 2. The process of claim 1 wherein saidpitch is introduced as granulated pitch.
 3. The process of claim 1wherein said pitch is introduced as powdered pitch.
 4. The process ofclaim 1 wherein said pitch is introduced as pelletized pitch.
 5. Theprocess of claim 1 wherein said pitch is a synthetic mesophase orisotropic pitch.
 6. The process of claim 1 wherein said pitch is apetroleum derived mesophase or isotropic pitch.
 7. The process of claim1 wherein said pitch is a coal-derived mesophase or isotropic pitch. 8.The process of claim 1 wherein said pitch is a blend of pitches selectedfrom the group consisting of synthetic mesophase or isotropic pitch,petroleum derived mesophase or isotropic pitch, and coal derivedmesophase or isotropic pitch.
 9. The process of claim 1 wherein saidpitch is a solvated pitch.
 10. The process of claim 1 wherein saidpurging is effected by a vacuum step.
 11. The process of claim 1 whereinsaid purging is effected by an inert fluid.
 12. The process of claim 1wherein said vacuum is applied at less than 1 torr.
 13. The process ofclaim 1 wherein nitrogen is introduced as the inert fluid.
 14. Theprocess of claim 1 wherein said pitch is heated to a temperature in therange of about 500° C. to about 1000° C. to coke said pitch.
 15. Theprocess of claim 1 wherein said pitch is heated to a temperature ofabout 800° C. to coke said pitch.
 16. The process of claim 1 wherein thetemperature to coke said pitch is raised at a rate of no greater than 5°C. per minute.
 17. The process of claim 1 wherein said pitch is soakedat the coking temperature for at least 15 minutes to effect said coking.18. The process of claim 1 wherein said pitch is heated to a temperatureof about 630° C. to coke said pitch.
 19. The process of claim 1 whereinsaid pitch is heated to a temperature of about 50° C. to about 100° C.to coalesce said pitch.
 20. The process of claim 1 where said foam iscooled at a rate of approximately 1.5° C./min with the release ofpressure at a rate of approximately 2 psi/min.
 21. The process of claim1 further including the step of densifying said foam.
 22. The process ofclaim 1 wherein said phase change material is acetic acid.
 23. Theprocess of claim 1 wherein said phase change material is a paraffin wax.24. The process of claim 1 wherein said phase change material isgermanium.
 25. The process of claim 1 wherein said encasement materialis polyethylene.
 26. The process of claim 1 wherein said encasementmaterial is aluminum.
 27. The process of claim 1 wherein said encasementmaterial is a carbon-carbon composite.
 28. A carbon foam heat sinkproduct as produced by the process of claim
 1. 29. A process ofproducing a carbon foam heat sink comprising: selecting an appropriatemold shape and a mold composed of a material that the molten pitch doesnot wet; introducing said pitch to an appropriate level in the mold;purging the air from said mold to form a vacuum; heating said pitch to atemperature sufficient to coalesce said pitch into a liquid; releasingsaid vacuum and backfilling an inert fluid at a static pressure up toabout 1000 psi; heating said pitch to a temperature sufficient to cokethe pitch; and cooling said foam to room temperature and simultaneouslyreleasing said inert fluid; at least partially encasing said foam; andat least partially filling porous regions of said foam with a phasechange material.
 30. The process of claim 29 wherein said pitch isintroduced as granulated pitch.
 31. The process of claim 29 wherein saidpitch is introduced as powdered pitch.
 32. The process of claim 29wherein said pitch is introduced as pelletized pitch.
 33. The process ofclaim 29 wherein said pitch is a synthetic mesophase or isotropic pitch.34. The process of claim 29 wherein said pitch is a petroleum-derivedmesophase pitch.
 35. The process of claim 29 wherein said pitch is acoal-derived mesophase pitch.
 36. The process of claim 29 wherein saidmold is purged by a vacuum applied at less than 1 torr.
 37. The processof claim 29 wherein said mold is purged by an inert fluid beforeheating.
 38. The process of claim 29 wherein said phase change materialis acetic acid.
 39. The process of claim 29 wherein said phase changematerial is a paraffin wax.
 40. The process of claim 29 wherein saidphase change material is germanium.
 41. The process of claim 29 whereinsaid encasement material is polyethylene.
 42. The process of claim 29wherein said encasement material is aluminum.
 43. The process of claim29 wherein said encasement material is a carbon-carbon composite.
 44. Acarbon foam heat sink product as produced by the process of claim 29.45. A process of producing a carbon foam heat sink comprising: selectingan appropriate mold shape; introducing pitch to an appropriate level insaid mold; purging air from said mold to form a vacuum; heating saidpitch to a temperature sufficient to coalesce said pitch into a liquid;releasing said vacuum and backfilling an inert fluid at a staticpressure up to about 1000 psi; heating said pitch to a temperaturesufficient to cause gases to evolve and form carbon foam; heating saidcarbon foam to a temperature sufficient to coke the pitch; cooling saidcarbon foam to room temperature and simultaneously releasing said inertfluid; placing facesheets on the opposite sides of said carbon foam;adhering the facesheets to said carbon foam; at least partially encasingsaid carbon foam and facesheets; and at least partially filling porousregions of said carbon foam with a phase change material.
 46. Theprocess of claim 45 wherein the adhering of the facesheets to the carbonfoam is effected by a molding step.
 47. The process of claim 45 whereinthe adhering of the facesheets to the carbon foam is effected by acoating material.
 48. The process of claim 45 wherein said phase changematerial is acetic acid.
 49. The process of claim 45 wherein said phasechange material is a paraffin wax.
 50. The process of claim 45 whereinsaid phase change material is germanium.
 51. The process of claim 45wherein said encasement material is polyethylene.
 52. The process ofclaim 45 wherein said encasement material is aluminum.
 53. The processof claim 45 wherein said encasement material is a carbon-carboncomposite.
 54. The process of claim 45 wherein said facesheets materialis a carbon-carbon composite.
 55. A composite carbon foam heat sinkproduct produced by the process of claim
 45. 56. A carbon foamcontaining a phase change material in at least some of its pores. 57.The carbon foam of claim 56 wherein said phase change material isselected from the group consisting of water, acetic acid, paraffin wax,germanium, and germanium-silicon.
 58. The carbon foam of claim 56encased to prevent loss of the phase change material when in a non-solidstate.
 59. The carbon foam of claim 58 wherein said phase changematerial is selected from the group consisting of water, acetic acid,paraffin wax, germanium, and germanium-silicon.
 60. The carbon foam ofclaim 56 wherein the foam is an essentially graphitic carbon foam havinga bulk thermal conductivity greater than about 58 W/m·° K.
 61. Thecarbon foam of claim 60 having an open cell pore structure.
 62. Thecarbon foam of claim 61 wherein the open cell pore structure issubstantially comprised of ellipsoidal pores.
 63. The carbon foam ofclaim 62 characterized by an X-ray diffraction pattern substantially asdepicted in FIG.
 14. 64. The carbon foam of claim 62 characterized by anX-ray diffraction pattern having an average d002 spacing of about 0.336.65. The carbon foam of claim 58 wherein the foam is an essentiallygraphitic carbon foam having a bulk thermal conductivity from about 58W/m·° K to about 106 W/m·° K.
 66. The carbon foam of claim 65characterized by an X-ray diffraction pattern exhibiting relativelysharp doublet peaks at 2 q angles between 40 and 50 degrees.
 67. Thecarbon foam of claim 66 characterized by an open cell pore structuresubstantially comprised of ellipsoidal pores.
 68. The carbon foam ofclaim 67 further characterized by graphite substantially aligned alongthe axes of the cell walls.
 69. The carbon foam of claim 56 wherein thefoam is an essentially graphitic carbon foam having a specific thermalconductivity greater than about 109 W·cm³/m·° K·g.
 70. The carbon foamof claim 69 characterized by an open cell pore structure substantiallycomprised of pores whose planar cross-sectional images are substantiallycircular or elliptical.
 71. The carbon foam of claim 69 characterized byan X-ray diffraction pattern having an average d002 spacing of about0.336 and exhibiting relatively sharp doublet peaks at 2 q anglesbetween 40 and 50 degrees.
 72. The carbon foam of claim 56 wherein thefoam is an essentially graphitic carbon foam having a specific thermalconductivity from about 109 W·cm³/m·° K·g to about 200 W·cm³/m·° K·g.73. The carbon foam of claim 72 characterized by an X-ray diffractionpattern substantially as depicted in FIG.
 14. 74. The carbon foam ofclaim 72 having an open cell structure with graphite aligned along thecell wall axes, said carbon foam being derived from a mesophase pitch.75. The carbon foam of claim 72 derived from a mesophase pitch.
 76. Thecarbon foam of claim 56 wherein the foam is an essentially graphiticcarbon foam having a specific thermal conductivity greater than copper.77. The carbon foam of claim 76 characterized by an open cell porestructure substantially comprised of ellipsoidal pores.
 78. The carbonfoam of claim 77 characterized by an X-ray diffraction patternexhibiting relatively sharp doublet peaks at 2 q angles between 40 and50 degrees.
 79. The carbon foam of claim 78 derived from a mesophasepitch.
 80. The carbon foam of claim 56 wherein the foam is anessentially graphitic carbon foam having a specific thermal conductivitygreater than four times that of copper.
 81. The carbon foam of claim 80characterized by an X-ray diffraction pattern exhibiting relativelysharp doublet peaks at 2 q angles between 40 and 50 degrees and anaverage d002 spacing of about 0.336.
 82. The carbon foam of claim 80characterized by an open cell pore structure substantially comprised ofpores whose planar cross-sectional images are substantially circular orelliptical.
 83. The carbon foam of claim 82 derived from a mesophasepitch.
 84. The carbon foam of claim 83 characterized by an X-raydiffraction pattern substantially as depicted in FIG.
 14. 85. Atemperature control apparatus attached to a spacecraft in an externallocation, said apparatus comprising an encased carbon foam containing inat least some of its pores a phase change material that will (1) undergoa phase change at a temperature induced by solar radiant energy when inspace and (2) revert to its former state when not exposed to solarradiant energy when in space.
 86. The apparatus of claim 85 wherein saidphase change material commences to melt at a temperature induced by saidsolar radiant energy when in space and freezes when not exposed to solarradiant energy.
 87. The apparatus of claim 85 wherein said spacecraft isa satellite and the phase change material will (1) undergo a phasechange at a temperature induced by solar radiant energy when in orbitabout the earth and (2) reverts to its former state when no longerexposed to said solar radiant energy.
 88. The apparatus of claim 87wherein said phase change material commences to melt at a temperatureinduced by said solar radiant energy when in orbit about the earth andfreezes when no longer exposed to said solar radiant energy.
 89. Anapparatus for thawing frozen food comprising an encased carbon foamcontaining in at least some of its pores a phase change material thatfreezes at a temperature above 0° C. but is liquid at room temperature.90. A temperature control apparatus for aiding in maintaining thetemperature of an object in contact therewith below 1800° C. comprisingan encased carbon foam containing in at least some of its pores a phasechange material that melts at an elevated temperature above about 800°C. but does not vaporize below 1800° C.
 91. An apparatus as defined inclaim 90 wherein said phase change material melts between about 800° C.and 900° C.
 92. An apparatus as defined in claim 90 wherein said phasechange material comprises germanium.
 93. An apparatus as defined inclaim 90 wherein said phase change material comprises germanium-silicon.94. A temperature control apparatus for aiding in maintaining thetemperature of an object in contact therewith below 1200° C. comprisingan encased carbon foam containing in at least some of its pores a phasechange material that melts at an elevated temperature above about 800°C. but does not vaporize below 1200° C.
 95. An apparatus as defined inclaim 94 wherein said phase change material melts between about 800° C.and 900° C.
 96. An apparatus as defined in claim 94 wherein said phasechange material comprises germanium.
 97. An apparatus as defined inclaim 94 wherein said phase change material comprises germanium-silicon.98. The apparatus of claim 89 wherein the carbon foam is an essentiallygraphitic carbon foam having a bulk thermal conductivity from about 58W/m·° K to about 106 W/m·° K.
 99. The apparatus of claim 98 wherein thecarbon foam has an open cell pore structure.
 100. The apparatus of claim99 wherein the carbon foam is derived from a mesophase pitch.
 101. Theapparatus of claim 100 wherein the open cell pore structure of thecarbon foam is substantially comprised of ellipsoidal pores.
 102. Theapparatus of claim 101 wherein the carbon foam is characterized by anX-ray diffraction pattern substantially as depicted in FIG.
 14. 103. Theapparatus of claim 99 wherein the carbon foam is characterized by anX-ray diffraction pattern having an average d002 spacing of about 0.336.104. The apparatus of claim 85 wherein the carbon foam is an essentiallygraphitic carbon foam having a bulk thermal conductivity greater thanabout 58 W/m·° K.
 105. The apparatus of claim 104 wherein the carbonfoam is characterized by an X-ray diffraction pattern exhibitingrelatively sharp doublet peaks at 2 q angles between 40 and 50 degrees.106. The apparatus of claim 105 wherein the carbon foam is characterizedby an open cell pore structure substantially comprised of ellipsoidalpores.
 107. The apparatus of claim 106 wherein the carbon foam isfurther characterized by graphite substantially aligned along the axesof the cell walls.
 108. The apparatus of claim 89 wherein the carbonfoam is an essentially graphitic carbon foam having a specific thermalconductivity greater than copper.
 109. The apparatus of claim 108wherein the carbon foam is characterized by an open cell pore structuresubstantially comprised of pores whose planar cross-sectional images aresubstantially circular or elliptical.
 110. The apparatus of claim 109wherein the carbon foam is characterized by an X-ray diffraction patternhaving an average d002 spacing of about 0.336 and exhibiting relativelysharp doublet peaks at 2 q angles between 40 and 50 degrees.
 111. Theapparatus of claim 85 wherein the carbon foam is an essentiallygraphitic carbon foam having a specific thermal conductivity greaterthan about 109 W·cm³/m·° K·g.
 112. The apparatus of claim 111 whereinthe carbon foam is characterized by an X-ray diffraction patternsubstantially as depicted in FIG.
 14. 113. The apparatus of claim 111wherein the carbon foam has an open cell structure with graphite alignedalong the cell wall axes.
 114. The apparatus of claim 113 wherein thecarbon foam is derived from a mesophase pitch.
 115. The apparatus ofclaim 111 wherein the carbon foam is derived from a mesophase pitch.116. The apparatus of claim 89 wherein the carbon foam is an essentiallygraphitic carbon foam having a specific thermal conductivity greaterthan four times that of copper.
 117. The apparatus of claim 116 whereinthe carbon foam is characterized by an open cell pore structuresubstantially comprised of ellipsoidal pores.
 118. The apparatus ofclaim 117 wherein the carbon foam is characterized by an X-raydiffraction pattern exhibiting relatively sharp doublet peaks at 2 qangles between 40 and 50 degrees.
 119. The apparatus of claim 118wherein the carbon foam is derived from a mesophase pitch.
 120. Theapparatus of claim 85 wherein the carbon foam is an essentiallygraphitic carbon foam having a specific thermal conductivity from about109 W·cm³/m·° K·g to about 200 W·cm³/m·° K·g.
 121. The apparatus ofclaim 120 wherein the carbon foam is characterized by an X-raydiffraction pattern exhibiting relatively sharp doublet peaks at 2 qangles between 40 and 50 degrees and an average d002 spacing of about0.336.
 122. The apparatus of claim 120 wherein the carbon foam ischaracterized by an open cell pore structure substantially comprised ofpores whose planar cross-sectional images are substantially circular orelliptical.
 123. The apparatus of claim 122 wherein the carbon foamderived from a mesophase pitch.
 124. The apparatus of claim 123 whereinthe carbon foam is characterized by an X-ray diffraction patternsubstantially as depicted in FIG.
 14. 125. The apparatus of claim 89wherein the carbon foam is a non-oxidatively stabilized, essentiallygraphitic carbon foam derived from a mesophase pitch, the carbon foamhaving an open cell structure and a specific thermal conductivitygreater than copper.
 126. The apparatus of claim 125 wherein the carbonfoam has a specific thermal conductivity greater than four times that ofcopper.
 127. The apparatus of claim 125 wherein the carbon foam ischaracterized by an X-ray diffraction pattern having an average d002spacing of about 0.336.
 128. The carbon foam of claim 56 wherein thecarbon foam is a non-oxidatively stabilized, essentially graphiticcarbon foam derived from a mesophase pitch, the carbon foam having anopen cell structure and a specific thermal conductivity greater thancopper.
 129. The carbon foam of claim 128 having a specific thermalconductivity greater than four times that of copper.
 130. The carbonfoam of claim 129 characterized by an X-ray diffraction pattern havingan average d002 spacing of about 0.336.
 131. The carbon foam of claim 56wherein the carbon foam is a non-oxidatively stabilized, essentiallygraphitic carbon foam derived from a mesophase pitch, the carbon foamhaving an open cell structure and a bulk thermal conductivity from about58 W/m·K to about 106 W/m·° K.